Sensor devices, including accelerometers, based on capacitive pick-off and electrostatic closed-loop rebalance are generally well known.
FIG. 1 illustrates, in accordance with prior art, a capacitive pick-off sensor constructed as a conventional mid-pendulum hinged or “teeter-totter” type accelerometer. Such devices are constructed using microcircuit techniques to produce reliable, maintenance-free capacitive acceleration-sensing devices. Such a capacitive acceleration sensing device 1, hereinafter a capacitive accelerometer, includes a pair of stationary substrates 2, 3 having opposed parallel planar faces. The substrates 2, 3 are spaced from one another and each has a number of metal electrode layers 4, 5 of predetermined configuration deposited on one surface to form respective capacitor electrodes or “plates.” The electrode elements 4 (or 5) operates as an excitation electrode to receive stimulating signals, and the other electrode elements 5 (or 4) operate as the feedback electrodes for electrostatic rebalance. A single set of electrode elements 4 (or 5) operates as both excitation and feedback electrodes when the feedback signal is superimposed on the excitation signal.
A pendulous acceleration sensing element 7, commonly referred to as either a “pendulum” or a “proof mass,” which operates as pick-off electrode, is flexibly suspended between the substrates 2, 3 at elevated attachment points 8 for pendulous rotation about a hinge axis h to form different sets of capacitors with electrode elements 4, 5. Movement of the acceleration-sensing element, or “pendulum,” 7 in response to acceleration changes its position relative to the stationary excitation electrodes 4 (or 5), thereby causing a change in pick-off capacitance. This change in pick-off capacitance is indicative of acceleration. A set of capacitors for electrostatic rebalance is made up of the sensing element 7 and the feedback electrodes 5 (or 4) for driving the sensing element 7 to its reference position balanced between the electrode elements 4, 5 and maintaining it there.
In such an acceleration sensor device, a capacitance formed by the excitation electrodes 4 (or 5) and the moveable sensing element 7 is related to 1/D, where D is the offset between electrodes 4, 5 and the hinge axis h of the pendulous acceleration sensing element 7 when constructed or emplaced relative to the substrates 2, 3 on the elevated attachment points 8.
A desirable characteristic of an accelerometer is a linear response for pick-off capacitance C versus acceleration input g. However, conventional high-g range teeter-totter type accelerometers have less than optimum linearity for high performance application and may also have a non-monotonic response for electrostatic rebalance force versus acceleration when feedback voltage is capped. The capacitance seen by the pick-off electrodes is related to the integral of 1/d(i) for each a(i) over the area of the excitation electrodes, where d(i) is the dynamic separation distance between the stationary electrodes and the pendulum for each incremental area a(i). The sensor's dynamic range, scale factor and response linearity are thus defined by the separation distance D (shown in FIG. 1) between the stationary electrode elements 4, 5 and the hinge axis h of the pendulous acceleration-sensing element 7, and the lateral offset of electrode elements 4, 5 relative to the attachment points 8. In a conventional teeter-totter type acceleration sensor device, the stationary capacitor electrodes 4, 5 are traditionally arranged substantially along a longitudinal axis of symmetry L of the acceleration sensing device 1 perpendicular to the hinge axis h of the acceleration-sensing element 7, as illustrated in FIG. 1. Electrode elements 4, 5 are sized and spaced symmetrically with respect to the longitudinal axis L of the acceleration sensing device 1, while the electrode elements 4 (or 5) operating as excitation electrodes are further sized and spaced symmetrically with respect to the attachment points 8 and the hinge axis h of the moveable sensing element 7.
Conventional teeter-totter type acceleration sensor devices of the type illustrated in FIG. 1 have been fabricated from a body of semiconductor material, such as silicon, as Micro Electro-Mechanical Systems, or “MEMS,” integrated micro devices or systems combining electrical and mechanical components fabricated using integrated circuit (IC) batch processing techniques.
In the most general form, MEMS consist of mechanical microstructures, microsensors, microactuators and electronics integrated in the same environment, i.e., on a silicon chip. MEMS is an enabling technology in the field of solid-state transducers, i.e., sensors and actuators. The MEMS microfabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks but in combination can accomplish complicated functions. Current applications include accelerometers, pressure, chemical and flow sensors, micro-optics, optical scanners, and fluid pumps. For example, one micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove unmasked portions thereof. The resulting three-dimensional silicon structure functions as a miniature mechanical force sensing device, such as an accelerometer that includes a proof mass suspended by a flexure. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. No. 5,006,487, “METHOD OF MAKING AN ELECTROSTATIC SILICON ACCELEROMETER”; U.S. Pat. No. 4,945,765 “SILICON MICROMACHINED ACCELEROMETER”; and co-pending U.S. patent application Ser. No. 10/368,160, “MEMS ENHANCED CAPACITIVE PICK-OFF AND ELECTROSTATIC REBALANCE ELECTRODE PLACEMENT” filed in the names of Aiwu Yue and Ronald B. Leonardson on Feb. 18, 2003, the complete disclosures of all of which are incorporated herein by reference.
High aspect ratio MEMS or “HIMEMS” is one known process for producing such MEMS devices, including MEMS accelerometer devices. HIMEMS permits fabrication of intricate device designs in two dimensions, but requires a fixed device thickness, on the order of a cookie cutter.
Acceleration sensors fabricated using MEMS or HIMEMS technology generally include a moveable sensing element of the type illustrated in FIG. 1 and indicated by the reference character 7. The moveable sensing element is attached through attachment points of the type illustrated in FIG. 1 and indicated by the reference character 8 to a lower plate of the type illustrated in FIG. 1 and indicated by the reference character 3, where the lower plate is a substrate on which moveable sensing element is manufactured. The lower plate or substrate on which moveable sensing element is manufactured has formed thereon one set of the metal electrode layers of the type illustrated in FIG. 1 and indicated by the reference characters 4, 5.
According to the current state of the art for fabricating conventional teeter-totter type acceleration sensor devices of the type illustrated in FIG. 1 using MEMS or HIMEMS technology, the substrates 2, 3 are etched to form a “reverse mesa” or “valley” beneath the acceleration sensing element 7 except at the attachment points 8, whereby each of the attachment points 8 is formed as a “mesa” that is elevated relative to the bulk of the substrates 2, 3. The single etch step or operation thus constructs the attachment points 8 and releases the silicon acceleration sensing element 7 from the bulk of the substrate 2 (or 3) for operation.
During the single etch step, the remainder of the substrates 2, 3 is simultaneously formed with a substantially planar surface 9, 10, respectively, spaced by the distance D away from the acceleration sensing element 7 when emplaced. The etching of the substrates 2, 3 thus leaves attachment points 8 spaced above the substantially planar substrate surfaces 9, 10. Thus, when emplaced on the elevated attachment points 8, the acceleration sensing element 7 is spaced a short distance away from the substrate surfaces 9, 10 so that narrow gaps g1, g2 (best illustrated in FIG. 3), usually on the order of a few microns, for example on the order of 2–4 microns, wherein the acceleration sensing element 7 is free to move during operation are formed between the substrate surface 9 (or 10) and surfaces of the acceleration sensing element 7 on either side of the elevated attachment points 8.
When intended for operation as a teeter-totter type accelerometer of the type illustrated in FIG. 1, a first portion 11 of the moveable sensing element 7 on one side of the hinge axis h is formed with relatively greater mass than a second portion 12 on the other side of the hinge axis h to develop a desired amount of pendulosity. The greater mass of the first portion 11 may be developed by offsetting the hinge axis h relative to the longitudinal dimension of the sensing element 7, as illustrated in FIG. 1. In a device 1 fabricated using MEMS or HIMEMS technology, the sensing element 7 is necessarily a substantially two-dimensional object of substantially uniform thickness so that offsetting the hinge axis h causes the longer first portion 11 to have relatively greater mass than the shorter second portion 12 with a center of mass spaced relatively further from the hinge axis h.
Offset of the hinge axis h to shift the mass and develop the desired pendulosity in the sensing element 7 is problematic in a device fabricated using MEMS or HIMEMS technology. The sensing element 7 is necessarily a substantially two-dimensional object of substantially uniform thickness. Therefore, a surface area 13 of an extended portion 14 near the distal edge or tip 15 of the first sensing element portion 11 that is extended further from the hinge axis h than the distal edge or tip 16 of the shorter sensing element portion 12 necessarily causes the overall surface area of the larger and more massive first portion 11 to be larger than the corresponding overall surface area of the smaller and less massive second portion 12. This difference in surface area between the first and second portions 11, 12 degrades performance when the accelerometer device is operated in a vibration environment.
Air or another gas trapped in the narrow gaps g1, g2 between the first and second portions 11, 12 of the sensing element 7 and the planar surfaces 9, 10 of the substrates 2, 3 provide gas damping of the sensing element 7 when the sensing device 1 is operated in a vibration environment. Because the gas damping gaps g1, g2 are very small the gas damping effects between the sensing element portions 11, 12 and the planar substrate surfaces 9, 10 is very sensitive to differences in the surface areas of the larger first and smaller second portions 11, 12 of the sensing element 7, and according to well-known laws of mechanics and fluid dynamics the gas damping effects are especially sensitive to the distances at which the disproportionate areas of the larger first and smaller second sensing element portions 11, 12 are offset relative to the hinge axis h. In other words, the gas damping effects are particularly sensitive to the amount by which the overall surface area of the first sensing element portion 11 is increased by the surface area 13 of the extended portion 14 near its distal edge or tip 15 relative to the overall surface area of the relatively shorter sensing element portion 12.
Performance degradation is realized as a vibration rectification error (VRE) that is the result of unbalanced gas damping in the gaps g1, g2 between the first and second sensing element portions 11, 12 and the planar substrate surfaces 9, 10. This problem is exacerbated in teeter-totter type accelerometers fabricated using MEMS or HIMEMS technology because the gas damping gaps g1, g2 are very small, usually only a few microns, and teeter-totter type designs are usually over-damped because of the large surface area relative to the very narrow gap.
It is known to balance the gas damping by perforating at least part of the longer first portion 11 of the sensing element 7 with a large number of small holes 17 through which the gas may be transferred. For example, the holes 17 are formed through the first and second sensing element portions 11, 12 (omitted for clarity) and the extended portion 14 (shown). These holes 17 also aid in transfer of chemicals during fabrication. While these small gas transfer holes 17 help alleviate the unbalanced gas damping and resultant VRE, the holes 17 also the reduce mass of the sensing element 7. The reduced mass in turn reduces the pendulosity of the sensing element 7, which is also important to performance.
As is well-known in the art, the operating range of an accelerometer of the type illustrated in FIG. 1 is physically limited to the acceleration that overcomes the ability of the device to electrostatically balance the sensing element 7 relative to the electrode layers 4, 5 and causes the end or tip 15 and the corresponding extended portion 14 of the more massive first portion 11 of the teeter-totter type sensing element 7 to touch down on the surface 9 (or 10) of the substrate 2 (or 3). When this happens, the device 1 stops working until the sensing element 7 comes off the substrate surface 9 (or 10).
Another limitation of teeter-totter type accelerometer devices fabricated using MEMS or HIMEMS technology is caused by the substantially uniform thickness and planar surfaces of components that is required by the MEMS and HIMEMS technologies. The extremely narrow gas damping gaps g1, g2 permit a substantial portion of the smooth, uniform and planar surfaces of the sensing element 7 to physically contact the smooth, uniform and planar surfaces 9, 10 of one of the first and second substrates 2, 3 when the accelerometer experiences an acceleration in excess of its operating range. Such substantial physical contact causes the damping gases to be expelled from the gap g. it is known in the art that spring and electrostatic rebalance forces of the accelerometer are not always sufficient to overcome the resultant static and vacuum effects of the high pressure contact between the smooth surfaces whereupon the sensing element 7 becomes irretrievably “stuck” to one of the substrate 2 (or 3), never to release from the substrate surface 9 (or 10).